[PATCH v2 1/4] drm/dp: Add helper to set LTTPRs in transparent mode

[Dmitry Baryshkov]

On Wed, Dec 11, 2024 at 03:04:12PM +0200, Abel Vesa wrote:
> According to the DisplayPort standard, LTTPRs have two operating
> modes:
>  - non-transparent - it replies to DPCD LTTPR field specific AUX
>    requests, while passes through all other AUX requests
>  - transparent - it passes through all AUX requests.
> 
> Switching between this two modes is done by the DPTX by issuing
> an AUX write to the DPCD PHY_REPEATER_MODE register.
> 
> Add a generic helper that allows switching between these modes.
> 
> Also add a generic wrapper for the helper that handles the explicit
> disabling of non-transparent mode and its disable->enable sequence
> mentioned in the DP Standard v2.0 section 3.6.6.1. Do this in order
> to move this handling out of the vendor specific driver implementation
> into the generic framework.
> 
> Signed-off-by: Abel Vesa <abel.vesa at linaro.org>
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 50 +++++++++++++++++++++++++++++++++
>  include/drm/display/drm_dp_helper.h     |  2 ++
>  2 files changed, 52 insertions(+)
> 


> +/**
> + * drm_dp_lttpr_init - init LTTPR transparency mode according to DP standard
> + *
> + * @aux: DisplayPort AUX channel
> + * @lttpr_count: Number of LTTPRs
> + *
> + * Returns 0 on success or a negative error code on failure.
> + */
> +int drm_dp_lttpr_init(struct drm_dp_aux *aux, int lttpr_count)
> +{
> +	if (!lttpr_count)
> +		return 0;
> +
> +	/*
> +	 * See DP Standard v2.0 3.6.6.1 about the explicit disabling of
> +	 * non-transparent mode and the disable->enable non-transparent mode
> +	 * sequence.
> +	 */
> +	drm_dp_lttpr_set_transparent_mode(aux, true);
> +
> +	if (lttpr_count > 0 && !drm_dp_lttpr_set_transparent_mode(aux, false))
> +		return 0;
> +
> +	/*
> +	 * Roll-back to tranparent mode if setting non-tranparent mode failed or
> +	 * the number of LTTPRs is invalid
> +	 */
> +	drm_dp_lttpr_set_transparent_mode(aux, true);

This means that if lttpr_count < 0, then there will be two requests to
set LTTPRs to a transparent mode. Is that expected?

> +
> +	return -EINVAL;
> +}
> +EXPORT_SYMBOL(drm_dp_lttpr_init);
> +

-- 
With best wishes
Dmitry